1. Field of the Invention
This invention relates generally to a switched capacitor and, more particularly, to a switched capacitor including a technique for switching the input of the switched capacitor between an input potential and a potential proportional to the input potential so as to increase the effective resistance.
2. Discussion of the Related Art
A switched capacitor is a capacitor, well known in the art, that is rapidly switched between different voltage potentials in order to repeatedly charge and discharge the capacitor so as to mimic a resistor. The typical prior art switched capacitor includes an associated input switch and output switch where the input switch is switched between an input potential and a ground potential, and the output switch is switched between an output potential and a ground potential at the same switching rate (see FIG. 1). In an integrated circuit application, the input and output switches are generally metal oxide field effect transistors. For a more complete background discussion of switched capacitors, see for example, Gregorian & Temes, "ANALOG MOS INTEGRATED CIRCUITS" Wiley-Interscience 1986.
Switched capacitors provide a number of advantages for integrated circuit applications. When fabricating an integrated circuit, resistors having accurate resistive values are sometimes difficult to achieve. Additionally, integrated circuit resistors having relatively large values are sometimes impractical to implement. An integrated circuit switched capacitor can be fabricated to provide accurate resistive values and can significantly reduce the chip area required by eliminating high value resistors and the requirement for off-chip resistors.
Another advantage in replacing resistors with switched capacitors is the ability to more closely match the resistive and capacitive components in order to provide a more reliable time constant of an integrated circuit. Because the resistors and capacitors of an integrated circuit are fabricated at different times during the fabrication process, the ability to match the values of these components so as to achieve a predictable time constant of the circuit is difficult. Therefore, a predictable time constant of the circuit cannot be accurately determined in that the time constant may have a wide range of error. As is well understood, this variance in time constants of an electrical circuit becomes a significant problem in certain applications.
Switched capacitors have been effective in matching resistive and capacitive components so as to narrow the predictable time constant of a particular electrical circuit. However, in order to generate relatively long time constants, high resistances are required. The effective resistance which can be established by a switched capacitor is limited by the switching frequency of the switches and the smallest practical capacitor size. A higher effective resistance is achieved by reducing the switching frequency and/or reducing the capacitor size. Therefore, the effective resistance is limited by practical switching frequencies and capacitor fabrication techniques.
The above described limitations present a significant drawback to switched capacitor technology for systems having relatively long time constants. What is needed then is a technique for increasing the effective resistance of a switched capacitor in order to realize these longer time constants. It is therefore an object of the present invention to provide such a switched capacitor.